Strapping machines of the type described are known and are used to strap, that is, to encircle, a package, for instance a stack of catalogs, with a plastic strap. Basically, a strap comprising plastic is guided about the package, tensioned, clamped and cut, and welded by heating by means of a welding device, whereupon the strapped package is conveyed out of the strapping machine and a new package to be strapped is supplied.
A strapping machine has a strap guide frame into which the strap is fed via a feeding device, and a strap guide in the area beneath the package in which the strap to be fed is guided and in which the leading strap end is captured. For tensioning the strap, it can be brought out of the strap guide frame and positioned directly on the package. Furthermore provided, is a clamping device comprising two separate clamping jaws, of which the one clamping jaw clamps the leading strap end and, after tensioning, the other clamping jaw clamps the other strap end, both disposed adjacent to an overlap area in which the welding occurs. Furthermore provided is a welding device including a welding tongue that is moved between the strap ends in the overlap area for heating them and a pressing device and a counterpressure plate, the pressing device being moved against the counter-pressure plate from below, and the heated strap ends that are to be welded being disposed therebetween. The movable functional components are controlled via a drive motor or a drive shaft, driven thereby, with which the different functional components are directly or indirectly motion-coupled. Frequently cam disk couplings are used, that is, one or a plurality of cam disks are arranged on the drive shaft and run at or on the corresponding catch segment of a functional component and are moved correspondingly as a function of the cam disk profile.
The drive shaft rotates 360° one time during a strapping cycle. In known machines, in the start position, that is, at the beginning of a strapping cycle, the new package to be strapped is already disposed in the strapping machine, that is, in the strap guide frame, the movable strap guide is disposed in the inserted work position, that is, it can guide the strap, and the strap itself has already been fed in and captured at its leading end. At this point in time, the drive motor is off, so the drive shaft is not rotating. Then the motor is started in a first work step, and the so-called set-up phase begins. The motor starts up in order to actuate the right clamping jaw of the clamping device via the drive shaft so that this clamping jaw clamps the leading strap end. As soon as this has occurred, the motor brakes again and stops, because then the tensioning step occurs next, which means that the strap, which naturally was fed in with a strap length significantly longer than the length required for strapping, is guided back into the machine into a strap storage unit and tensioned securely about the package. During this time the motor, or the drive shaft, is idle, because the tensioning process is performed by a separate tensioning device. Only then does the motor or drive shaft start up again in order to actuate the left clamping jaw in the next work step to clamp the strap at the other end, now cut, and in order to actuate the other functional components for the welding (for this the strap guide is moved out of the work area, the welding device is moved into the work area, after heating it is moved back out, and the pressing device is moved against the pressure plate to perform the welding). After the welding has concluded, while the motor drive is still running, the pressing device is moved back into the starting position and the pressure plate is moved out of the work area so that the package, now strapped, is exposed and can be exchanged for another package. As soon as the strapped package has been removed, the counterpressure plate is moved back into the work position together with the strap guide, and the motor stops—its work cycle has concluded. This is just followed by the strap for the next strapping process being fed in. The motor work cycle has concluded with the second stop, however, the motor or the drive shaft has rotated 360° one time. Then the new motor work cycle begins, namely, when the motor briefly turns again in order to actuate the right clamping jaw, and it stops again immediately thereafter because the tensioning process begins then as described in the foregoing.
It can be seen that a two-stop method is created in the known strapping machines. The motor starts up two times within one cycle and stops two times, the first start-up in the set-up phase being used only to actuate the right clamping jaw for fixing the leading strap end, whereupon the motor stops to enable the tensioning. This is disadvantageous not only for reasons of wear, but it also increases the time required for strapping one package so that machine throughput is necessarily reduced because of this.
The underlying problem of the invention is to provide a method that facilitates more rapid strapping and leads to less wear.